1. Field of the Invention
The present invention relates to a nuclear magnetic resonance imaging apparatus for obtaining tomographic images of a body to be examined by utilizing a phenomenon of nuclear magnetic resonance.
2. Description of the Background Art
A nuclear magnetic resonance imaging apparatus provides an important means for analyzing atomic and electronic structures of matter or chemical compounds in a solid state and an organic chemistry research. Also, there are increasing uses in medical practice of the nuclear magnetic resonance imaging apparatus as a diagnostic device for obtaining tomographic images of arbitrary cross sections of a body to be examined, on a basis of such information as hydrogen distribution and spin relaxation times extracted by utilizing a nuclear magnetic resonance phenomenon.
A main portion of a conventional nuclear magnetic resonance imaging apparatus is shown in perspective view and in cross sectional view in FIGS. 1 and 2, respectively. As shown, there is a main magnet 1 with bore for generating a static magnetic field, a hollow cylindrical outer shell 2 inside the bore which is connected to the main magnet 1 by supporting member 3, and a hollow cylindrical inner shell 7 inside the outer shell 2 which is connected to the outer shell 2 by ring members 8. Between the outer shell 2 and the inner shell 7, there is a gradient coil 21 for producing gradient magnetic field which is connected to the outer shell 2 by means of first gradient coil supporting member 4, a second gradient coil supporting member 5 and a support rubber 6.
As shown in FIG. 3, the gradient coil 21 comprises a X-coil 25, a Y-coil 27, and a Z-coil 29 wound around a coil core 23, each of which produces the gradient field in X-, Y, and Z-directions, respectively. Also, as shown in FIG. 4, these X-coil 25, Y-coil 27, and Z-coil 29 are fixed on the coil core 23 by molding with a non-magnetic resin 24 with a relatively large Young'3 s modulus, such as epoxy resin.
Each of these X-coil 25, Y-coil 27, and Z-coil 29 is provided with a separate power source, so that pulsed current for producing the gradient field can be applied separately when taking measurements.
However, as such gradient coil 21 is used in a presence of a very large static magnetic field generated by the main magnet 1 (typically between 0.22 to 1.5 Tesla), a considerable amount of electromagneetic force is exerted on each of these X-coil 25, Y-coil 27, and Z-coil 29, which gives vibrations of the gradient coil 21, which in turn causes the large acoustic noise.
Although in the nuclear magnetic resonance imaging apparatus of FIG. 1 the gradient coil 21 is confined inside a space formed by the outer shell 2, the inner shell 7 and the ring members 8 so as to muffle such acoustic noise, there still is a significant amount of vibration of the outer shell 2 caused by the vibration of the gradient coil 21 mediated through the air and through the first gradient coil supporting member 4, the second gradient coil supporting member 5 and the support rubber 6, as well as through the ring members 8. The noises may also be produced by the vibration of the coil core 23 and the resin 24.
Such acoustic noises can be quite disturbing to a patient to be examined who will be placed in a measurement space inside the inner shell 7.
Thus, in a conventional nuclear magnetic resonance imaging apparatus it has not been possible to eliminate all the disturbing acoustic noises originating from the vibration of the gradient coil 21.